In a spinning or other textile machine, it is common to provide a drafting frame which may comprise a set of lower rollers or a combination of such rollers and, for example, a moving belt with pressurizing arms which carry upper rollers and/or belts which can press a textile strand to be drafted in the frame against the lower roller and/or belt so that, depending upon the speeds of the rollers or belts successively engaging the strands, the latter can be drawn out or drafted. Drafting frames can be used between the cans supplying the sliver to the roving flyers or spindles in the winding of the roving on bobbins. They can also be provided in the ring spinning or other spinning frames to draft the roving, yarn or other textile strand.
The arms can be pressed toward the lower rollers and/or belt by pneumatic effectors and it is known to provide a system for pressurizing such effectors, which can be piston and cylinder units or an expandable tube with compressed air from a pressure source such as an air compressor through a valve which controls the pressure via a line delivering the compressed air to the pneumatic effectors. A pressure measuring device or sensor can be connected to the line which can also have a flowmeter and a control means for adjusting the valve in response to the pressure and/or the flow.
A system of this type is described in German Utility Model 296 09 348.3. In this system, the flowmeter is a float which is received in a conical tube and whose height can be detected by a sensor alongside the conical tube. Since the leakage loss which can be tolerated and the flow rate of the apparatus are dependent upon the operating pressure, the setting of the sensor must be matched in case of a change in operating pressure. When this matching is not effected, high leakage losses may prevent the desired operating pressure from being achieved. As a result, the requisite pressure at the drafting rollers may not be sufficient and the quality of the yarn which is produced will deteriorate. The importance of such matching will be clear from the fact that with operating pressure of 1.5 to 4 bar in the line supplying the effectors, a deviation of as little as 0.3 bar will result in a fluctuation in the yarn character.
In the German Patent Document 296 09 348.3, moreover, it has been indicated that other flowmeters can be substituted, but there is no suggestion as to how the flow parameters should be varied for different operating pressures or how it might be possible to avoid detrimental fluctuations in the spinning parameters at least to the extent that the system is not overseen continuously be service personnel.